1. Field of the Invention
The present invention relates to a control device for an AC generator.
2. Description of the Related Art
FIG. 3 is a circuit diagram of a conventional control device for an AC generator in the form of a vehicular AC generator. This control device comprises a rectifier 2 provided on the output side of an AC generator 1, a voltage regulator 3 for controlling the AC generator 1, a storage battery 4 which is charged by the output voltage of the AC generator 1, a key switch 5 which is operated when an engine (not shown) is to be started, and a charge indicating lamp 6 connected with the key switch 5 in series and between the storage battery 4 and an indicating-lamp drive terminal a of the voltage regulator 3 for indicating that the AC generator 1 is generating electricity normally.
The AC generator 1 comprising an armature coil 101 and a field coil 102 is driven by the engine. The rectifier 2 is a full-wave rectifier which full-wave rectifies the AC output voltage of the AC generator 1. The rectifier 2 has a main output terminal 201 and a negative output terminal 202, which are rectifier output terminals for outputting a main output.
The voltage regulator 3 comprises transistors 313 and 314 as switching elements, and resistors 301 and 302, all of which constitute a power-supply drive circuit. The voltage regulator 3 further comprises a diode 311, a transistor 312, and a base resistor 303 of the transistor 312, all of which constitute a control circuit for the charge indicating lamp 6.
Further, the voltage regulator 3 comprises a resistor 304 for detecting the one-phase voltage of the AC generator 1, a smoothing capacitor 317, a Zener diode 316 for determining the one-phase voltage level of the AC generator 1, and a transistor 315 for controlling the transistor 312 based on the output of the Zener diode 316.
Also, the voltage regulator 3 comprises voltage dividing resistors 305 and 306 for dividing an output voltage of the AC generator 1 to detect the output voltage, a Zener diode 319 with its cathode connected between the voltage dividing resistors 305 and 306, a transistor 320 which is connected at its base with the anode of the Zener diode 319 and turned on and off in accordance with whether the Zener diode 319 is on or off, an output transistor 321 which is turned on and off by the transistor 320 to control the field current of the field coil 102, and a base resistor 307 of the output transistor 321. Further, the voltage regulator 3 is connected in parallel to the field coil 102 and provided with a suppression diode 322 for absorbing intermittent surges occurring in the field coil 102.
The voltage dividing resistors 305 and 306, the Zener diode 319, the transistors 320 and 321, and the base resistor 307 as a whole constitute an output-voltage regulating circuit for detecting an output voltage of the AC generator 1 and intermittently controlling a field current to the field coil 102 to regulate the AC generator output voltage to a predetermined value.
Next, the operation will be described. If the key switch 5 is closed in starting the engine, a current will flow from the storage battery 4 through the key switch 5, the charge indicating lamp 6, and the indicating-lamp drive terminal a and from the resistor 301 to the base of the transistor 313, turning the transistor 313 on. When the transistor 313 is turned on, a base current also flows through the resistor 302 to the transistor 314 to thereby turn it on, so that current is supplied from the storage battery 4 to each part. At this time, a base current flows to the transistor 312 through the resistor 303, so the transistor 312 is turned on and the charge indicating lamp 6 lights. This lighting of the charge indicating lamp 6 indicates that in this state the AC generator 1 has not yet begun to generate electricity and is in its non-generating state. If, on the other hand, the base current flows to the output transistor 321 through the resistor 307, the output transistor 321 will be turned on, so an excitation current will flow from the storage battery 4 to the field coil 102, enabling the AC generator 1 to generate electricity.
Next, if the AC generator 1 starts generating electricity after start of the engine, the one-phase voltage of the AC generator 1 will rise. This one-phase voltage is detected through the resistor 304 and smoothed by the smoothing capacitor 317. If this smoothed average voltage exceeds the Zener voltage of the Zener diode 316, the transistor 315 will be turned on and the transistor 312 will be interrupted, so the charge indicating lamp 6 will be turned off. This off state of the charge indicating lamp 6 indicates that the AC generator I has begun generating electricity normally.
Also, with the start of electricity generation by the AC generator 1, the voltage of the main output terminal 201 of the rectifier 2 will also rise. This output voltage is detected through the transistor 314 with the voltage dividing resistors 305 and 306. If that output voltage exceeds a predetermined value set by the voltage dividing resistors 305 and 306 and the Zener diode 319, the Zener diode 319 will be turned on and the transistor 320 will be turned on. If, on the other hand, that output voltage is below the predetermined value, the Zener diode 319 will be turned off and the transistor 320 will also be turned off. Thus, if the transistor 320 is turned off, the output transistor 321 is also turned off and the field current passing through the field coil 102 is turned on and off, so the output voltage of the AC generator 1 is regulated to the predetermined value. Further, any intermittent surge occurring in the field coil 102 when the field current is turned on and off is absorbed by the suppression diode 322.
Since the conventional control device of the vehicular AC generator has been constructed as described above, the transistors 313 and 314 of the power-supply drive circuit, which are switching elements, are in the shut-off state at all times and cannot supply the base current to the output transistor 321, when the charge indicating lamp 6 is broken due to vibration, long use or the like, or the indicating-lamp drive terminal a is disconnected. Therefore, since the output transistor 321 remains off at all times and the AC generator 1 is prevented from generating electricity, there was the problem that the charging of the storage battery 4 becomes impossible, the storage battery 4 finally becomes dead, and engine operation is halted.